The gastrointestinal absorption of 63Ni and 95Nb has been determined in adult and neonatal rats after administration as various chemical forms. For 63Ni, f1 was determined as (4.3 ± 1.4) x 10-2, (5.1 ± 2.1) x 10-2 and (3.3 ± 1.5) x 10-2, for the citrate, oxalate and chloride forms, respectively. For 95Nb, the corresponding f1 values were (1.25 ± 0.37) x 10-2, (0.37 ± 0.15) x 10-2 and (0.24 ± 0.07) x 10-2, respectively. When radionuclides were administered to 1 day old animals, the retention increased by factors of up to 40 and 200 for 63Ni-chloride and 95Nb-oxalate, respectively. All these results are discussed with respect to the ICRP recommendations.